Can being over meticulous with painting cause earthing issues? For example the horn push on the handlebar - it has a single wire on the harness, so must get earth from the frame...so if I've gone and painted the whole thing, am I likely to have problems? Same goes for the engine/sparkplug I guess, I'm hoping the spark plug has property bitten through the paint on the head to ensure conductivity all the way through to the mag.

Has anyone ever had any trouble here?

email (option): Adh325@hotmail.com

big time...i ended up running extra earth wires all over,

email (option): chris.astinbarker@btinternet.com

Andy, sounds like we are working in parallel.
I fitted new horn and dip switches recently and was playing around seeing where the horn button would and wouldn't work - placed on a well painted surface it didn't work. My bars are chromed so no problem for me there though.
I can't see the spark plug being an issue as you have the metal to metal contact on the threaded portion.
I have also run extra earth wires in an effort to minimise losses in the elec system - important in my case as i am running the small 16H dynamo at 12V. Time will tell whether that was a good idea.

I ran no extra earths on mine - originality and all that. It's certainly necessary to scrape the paint off the bars where that small nipple is on the horn button clamp, preferably with a penknife marked 'Sheffield, England'

Don't forget the earth connection to the grub screw on the magneto body.